Reciprocal circuit



c. A. MENELEY 43,540

July 9, E46.

RECIPROCAL CIRCUIT Filed April 23. 1945 2 sheets-sheet 1 Juy 1946. c. A. MENELEY REGIPROCAL CIRCUIT Filed April 23, 1943 2. Sheets-Sheet 2 www 1.2: 5.....

Gttorneg llllllnllulllllll lllnlllillllilnilnlll i i i l l L Patented July 9, 10946 RECIPROCAL CIRCUIT Carl A. Meneley, Princeton, N.l J., as'signor to Radio Corporation of America, -:incorporation off I Delaware Application April'23, 1943,'Se1ial'NoL 484,304'1' l My' invention relates to the production of the reciprocal of a signal or wave and particularly to the production of such a signal for use in a sys-f tem for secret communication.

In the copending application of Alda V. Bedford Serial No. 456,578, filed August 29, 1942, and entitled Secret communication system,l there is described a communication system in which the speech or other communication signal S is multiplied by a coding signal K and in which the product SK is transmitted. At ithe receiver the incoming signal SK is multiplied. by the reciprocal of the coding signal K to obtain the original signal S, that is, SK 1/K=S. According to the above Bedford application, the coding signal K may be generated-by means of acode disc which is cut toy make-its periphery have the wave form of the signal K lwhile the decoding signal l/K may be produced by, a differentA discwhich is cut in accordance with calculated values to make its periphery have the Wave form of l/K. In a later application of Alda V. Bedford Yand Frank P. Wip, Serial No.480,716, led March 26, 1943, and entitled Reciprocal circuits, there is described one type of electrical circuit for producing-the reciprocal ofa signal or wave such as the wave K. f y r' An object of the present-invention isl to pro-A vide an improved method of and means for producing the reciprocal of a signal or Wave.

A further object of the invention is to provide an improved electrical circuit for converting an applied signal having a certain wave form into a signal having the reciprocal wave form.

A still further object of the invention is'to provide an improved communication'system for secret signalling.

In a preferred embodiment of my invention the reciprocal of a wave K is-obtained yby means of an electrical circuit in which'the waveK is clipped on both its positive cycle and onits negative cycle to produce a rectangular wave, and in which the wave K and the rectangular'waveare added together with one of them' reversed in polarity, preferably after the peaksofthe positive and negative cycles of the wave K have been squashed or flattened somewhat. My reciprocal circuit contains nocapactive lor-ginductive reactances (the blocking capacitors inthecircuit presenting a negligible impedance and,; therefore, functions the same for )any applied signal regardless of its frequency or waveform. e

In utilizing the reciprocal lcircuit in a commu-- nicationy system,;a coding signalK--is generated at the transmitter in any suitable manner'fas'by s claims; (ci. ricami means ,ofga coding disc,-thespeech signaly S is multiplied'by the signal K,1and the signal SKf-is transmitted ,to the receiver. At theV receiver there is another signal generating device for pro.-r ducing the signal K; it may be a coding disclike that at the transmitter, the two coding discsbeingv runin synchronism. The signal'K generated at 'the` receiver is supplied to the .reciprocal circuitfto obtain the decoding signal `1/K,-which-is then multiplied With the received signal SK to obtain'the-Qrieinal signals-f: i 1 f, 11;

The invention Lwill be better understoodfrom ther following description taken vin connection with vthe accompanying'drawingsin which ,Figure `1 is a vblock diagram of signalling appar, ratus embodying my inventiom -f Figures 2 to 5 are' graphs which 'arereferredto in explaining the invention; i l A Figure 61isa circuit diagram of a reciprocal circuit designed inaccordance with one embodiment of the-invention, i Y v v- A. 'Figure 7 is a circuit diagram ofanother reciprocal circuit designed in` accordance with another embodiment ofthe invention, and A, --Figure 'is a` circuitdiagramiof-a multiplier circuit Vthat may be `utilized in- .the circuitioffFig. 1;

-Inthe several' gures; similar parts are indi cated by similar reference characters. g f

.i--Referring `to Fig; 1,V the invention is shown ap` plied to radio apparatus that may-be switchedgfoznf,

operation either as a transmitter or as a receiver. Theseveral switches are shown in the'ppsi-tionior transmitter operation, vThis is f the t operation f oi the apparatus that `will iflrst Vbe described. ,A microphone and a speechampliner are-showniat f'lyand v8,-respectively.,` 'The .signal S is applied through -a switch.k 9 to a multiplier unit. i4, which may be of; the samev designv as thatdescribedyin the above-identified. Bedford application or which maybe of the type vshownin Fig. -8 and described and `claimed ingapplication Serial No. 484,303, led on the same day as the present applicationin the namepf Frank-P. Wipff and entitled Multiplier Fcircuits. 'The code Isignal K may :be profl ducedfby; means of a'code disc I6, a maskA |71, a light source'l, a condensing lens I9, and a photoelectric-Y cell '2 0., ,The signalrK is, supplied -through amplifiers 2l and. 22 and through a switch to the multipler unit '14. The resulting multiplier-.output signal -SK is supplied through anv amplier 2B*- and through a sWitchZl ltoa.

radio-v transmitter 28 or to a wire line, if .preferred.`

' The receiver Vfo-rfdecocling the gsignalSK may C be the same apparatus. as rin Eig. :1.,with switches 9, 23 and 21 thrown to their contact positions R. The signal SK is supplied from a radio receiver 3| through the switch 9 to the multiplier I4. The coding signal K produced by the disc I6 is supplied through the amplifier 2l to a reciprocal circuit 32 designed in accordance with my invention and shown in detail in Fig. 6. The reciprocal circuit 32 supplies the decoding signal l/K through the switch 23 to the multiplier I4. The resulting output of multiplier I4 is the original communication signal S since SK l/K=S. The signal S is amplied by the amplifier 26 and supn plied through the switch 21 to headphones or to the loudspeaker 33.

The code disc I6 at the transmitter and the similar code disc at the receiver are held in synchronism and in the proper phase relation by suitable synchronizing means. For example, at the transmitter a 60G-cycle per second current from a source 36 may be supplied through a switch ,31 to a synchronous motor 38 which rotates the code disc I6. The GOO-cycle current also modulates a radio transmitter 39 for the transmission of synchronizing signals to the receiver. At the receiver the switch 31 is in its contact position R whereby the received 600- cycle current is supplied from a radio receiver 4I to the synchronous motor 38. In some cases it may be preferable to transmit the synchronizing signal over a wire line.

My method of obtaining the reciprocal wave I /K will now be explained with reference to the graphs of Figs. 2 to 5. The graph of Fig. 2 represents the wave K while the graph of Fig. represents substantially the reciprocal wave I/K which is the sum of the flattened wave m of reversed polarity shown in Fig. 3 and the rectangular wave n of Fig. 4. The squashed or flattened wave m may be obtained by passing the wave K through a circuit that changes its resistance with a change in applied voltage. The rectangular wave n may be produced by clipping the positive and negative cycles of the wave m at the voltage levels e and f, respectively, for example, near the alternating-current axis of the signal and then amplifying the clipped signal.

Fig. 6 shows, by way of example, one reciprocal circuit that -may be employed in practicing the invention. The wave K, having a peak-to-peak amplitude of 6() volts, for example, is applied through a blocking capacitor 5I and a resistor 52 to a copper oxide rectifier unit 53 which vfunctions as a non-linear resistor having the property of decreasing in resistance as the applied voltage increases. The resistor 52 is of high enough resistance so that the driving source for the non-linear resistance unit 53 is of high impedance whereby there is only a slight variation in the current iiow through the unit 53. The unit 53 may consist of a pair of copper oxide rectifiers 53a and 53h connected to conduct current in opposite directions.

The voltage appearing across the non-linear unit 53 is the voltage m of Fig. 3 having the flattened wave form. This voltage is amplified by a cathode biased vacuum tube 54 and appears across an anode resistor 56 and a portion of the anode resistor 51-51 of a second amplifier tube 58.

The rectangular wave n is produced, in this particular example, by applying the output of the tube 54 through a blocking capacitor 59 and a high Yimpedance resistor 6I to a pair of diodes 62 and 63 which are connected to conduct in opposite directions. Output resistors 64 and 66 of comparatively low resistance are connected in series with the diodes 62 and 63, respectively. A biasing voltage drop for opposing current ow through diode 63 is produced across the resistor 66 by connecting a source of voltage (not shown) thereacross, a resistor 61 being in series with the voltage source. The diodes 62 and 63 clip the applied wave m symmetrically about its A.C. axis because a voltage which causes current flow through the diode 62 and resistor 64 is built up across the capacitor 59 by the positive cycle pulses ilowing through the diode 63. Thus, the diodes 52 and 63 become conducting on alternate cycles when the signal voltage exceeds the D.C. voltage drop across the resistors 64 and 66, respectively. The resulting rectangular wave 1L is amplified and reversed in polarity by the tube 58. The wave n and the flattened wave m add in the portion of the anode resistor 51-51' that is common to the tubes 54 and 58 to produce the desired reciprocal wave I/K shown in Fig. 5.

If the wave m is flattened correctly and if the waves m and n are added with the correct relative amplitudes, the resulting signal will be substantially a true reciprocal of the wave K. The only substantial departure from a true reciprocal signal will be where the wave K crosses the A.C. axis. Here the reciprocal value is infinity whereas the maximum amplitude of the wave I/K necessarily has a denite limit. The .waves m and n may be mixed with the correct relative amplitudes by adjusting a variable tap '1l on the anode resistor 51. The correct shaping of the flattened wave m may be 'obtained byselecting a non-linear resistor unit 53 having a suitable voltage-resistance characteristic and by adjusting the value'of the resistor 52.

As previously noted, the above-described reciprocal circuit isV purelyresistive so that its operation is independent of frequency. The in` stantaneous voltage 'output of the circuit'A is always the reciprocal of the instantaneous applied voltage. It follows thatf the reciprocal circuit is adjusted to produce the reciprocal of an appliedI signal having one wave form, the circuit will then always produce the lreciprocal of an applied signal regardless of itsv wave form. There are various ways of determining when the circuit has been adjusted to give substantiallya true reciprocal` One way is to connect the reciprocal circuit into the signalling system of Fig. 1 and, while transmitting speech or music. adjust the resistor 52 and the variable tap 1I at the receiver until the speech or music has a minimum of distortion.

Fig. 'Z shows a reciprocal circuit that is the same as that of Fig. 6 except that a `pair of diodes 12 and 13 have been substituted for the copper oxide rectiers 53a and 53h. The diodes i2 and 13 may beproperly biased by adjusting a pair of variable taps 14 and 16 on resistors 11 and 18, respectively. When properly biased, the two diodes will be operated along the lower knee of their characteristic curve and in the proper region to shape the wave m in the desired man ner. It will be noted that the bias applied. to the cathode of the diode 12 is of positive polarity and corresponds tothe positive bias applied to diode 63 of Fig. 6. Also, the diode circuit 12- 13 operates the same as the diode circuit 62-53 except that the adjustment of the bias and the magnitude of the applied signal are such that the signal is not clipped. i A

It will be understood that the invention is not limited to the particular circuits illustrated vsince the lwaves m 'andr-n, maybe derived from "the wave K in various-'ways andsince the two 'waves may be combined by means of a variety of circuits.

According to the above-identied Wipff application, the multiplier circuit I4 may utilize' one of several known types of circuits that have the. property of providing an instantaneousoutput voltage which is proportional to the square of the instantaneous input voltage for a reason'- able voltage swing. Such circuits will be referred to as squaring circuits, and will be designated Q where referred to in the description anfd drawings. In the multiplier circuitillustratedin Fig. 8, the waves S and K to be lmultipliedl are each separately squared in two squaring-circuit's Q1 and Q2. In a branch circuit the signals S and K are added, then the sum S|K is squared intathird squaring circuit Qa and the squared signal is reversed in polarity. 'Ihe three outputs of Q i,

Ch and Q3 are then added to obtain the sum as shown in the following equations: 1' +2.84

The plate resistors for such tubes should be kept' small, not more than about 1000 ohms.

The signals S and K are applied to the grids of tubes 80 and 82 through resistors 83 and 84, respectively. Both of the signals S and K are applied to the grid of tube 8| through resistors 85 and 86, respectively. A suitable negative bias is applied to the grids of the tubes 80, 8| and 82 through grid resistors 81 and 88. The resistors B3, 84, 85, 86, 81 and 88 all have the same comparatively high resistance, such as between 0.1'

megohm and 1 megohm, so as not to load the comparatively low impedance driving source too much. However, the resistance is not made so high as to permitexcessive phase shift due' to grid-cathode capacity. Since the pairs of resistors 83-81, 85--86, and 84-88 act as voltage dividers for the applied signals, the voltages applied to the grids of tubes 80, 8| and 82 will be S-l-K K 2 Silld respectively.

The grid bias C) on tubes 80, 8| and 82 should be about to the middle of the straight portion of the grid voltage vs. mutual' conductance curve. For a 6SN7 or 6J5 with 250 volts on the plate, this is about -14 volts.

A vacuum tube 89 serves to reverse the polarity of the output of tube 8| and should have approximately unity gain and very low distortion. The outputs of the tubes 80 and 82 and of the reversing tube 89 are added by supplying them to a common junction point 9| through adding resistors 92, 93 and 94. These adding resistors may have a resistance of from 0.05 megohm to 0.1

megohm, for example. If everything isproperlyY "and microfarads.

adjusted; only.v .the f 'desiree product iterm'igzsx and the D.C. component A2. appear'at'the 'junction-point 9|. In practice it is necessary "toadju'st'the multiplier circuit carefully for minimum l residual fsig'nal when' eitherthe signal Sor'the signal K is removed:

' '-In the several figures, circuit values are'rgiven,

merely bywayof example, in ohms, megohms,

I `claim as my invention: f.

"1 The methcd 0f VI'Qdling the reciprocalv of :a lsignal that-reverses in polarityat intervals with 'respect to an alternating-current axis whereby the signal has'cross-ov'er'points yon saidfaxis, said niethodlcomprising the stepsof producing'a'si'g- "rial'that `has ai relatively largev amplitude substantially at thetime of occurrence -of said; crossoverpoints, and' whichireverseis in polarity rat said cross-over points, producing another'si'g'nal that has a wave form approximately,` similar to that of the first-mentioned signal, and adding the two produced signals with one of them reversed in polarity and having the proper amplitude with respect to the other produced signal for producing a sum voltage that is substantially the desired reciprocal signal.

2. The method of producing the reciprocal of a signal that reverses in polarity at intervals withv versed in polarity and having the proper ampli- ,k tude with respect to the other produced signal for producing a sum voltage that is substantially the desired reciprocal signal.

3. The method of obtaining the reciprocal of a, signal having a certain `wave' form which comprises the steps of producing a signal of` substantially rectangular wave form having positive and negative cycles corresponding to the positive and negative cycles of the first signal, producing another signal that is generally similar in vwavev form to the first-mentioned signal, and adding ,Y

said rectangular wave to said other wave with one of the waves reversed in polarity andV with the two waves having such relative amplitudes that said reciprocal signal is obtained? 4. The method of obtaining the reciprocal of 9, signal having a certain wave form which comprises the steps of producing a signal of substantially rectangular wave form having positive and negative cycles corresponding to the positive and negative cycles of the rst signal, flattening the y positivejand negative cycles of the '.rst signal, and Aadding said rectangular wave to saidilattened wave with one of the `waves reversed in polarity.

5. An electrical circuit for` obtaining Vthe reciprocal value ofa voltage applied thereto comprising means for producing a substantially rectangular wave having positive and negative cycles corresponding tothe positive and negative cycles of a voltage applied to said circuit, vI neans for producing a wave having flattened positive and negative cycles corresponding to the positive and negative cycles of the voltage applied to said circuit, and means for adding said rectangular wave Ito 'said iiattened Wave with ,one

ofthe waves reversed in polarity.

.6. The invention according to claim5 wherein lthe impedance of the circuit is substantially purely resistive at the-frequencies of the applied l voltage. u l Y 7. `An electrical circuit for obtaining the reciprocal of a signal voltage applied thereto comprising a non-linear resistive device which decreases in resistance with an increase in applied voltage, means for passing said signal voltage through said resistive device whereby its wave -form is flattened, means for producing a substantially rectangular voltage wave having positive and negative cycles corresponding to the positive and negative cycles of said signal wave, and means for adding said rectangular Wave to said flattened wave with one of said waves reversed in polarity.

8. Anelejctrical circuit for obtaining the reciprocal of a signal voltage applied thereto `comaprising a non-linear resistive device which de- CARL A. MENELEY. 

